Holographic information recording and/or reproducing apparatus

ABSTRACT

A holographic information recording and/or reproducing apparatus including: a light source unit emitting reference light and signal light; a first optical path guiding unit guiding the lights to cross; a second optical path guiding unit including a first polarization converter located on an optical path of one of the reference and signal lights, a first polarization beam splitter located at a crossing point of the reference light and the signal light, an optical path converter guiding the reference light and the signal light so that they cross again, a second polarization converter located on an optical path of the signal light before the signal light crosses the reference light, and a second polarization beam splitter uniting the optical paths of the reference light and the signal light; and an objective lens unit illuminating the reference light and the signal light onto one side of a holographic information storage medium.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No.2007-140557, filed on Dec. 28, 2007, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a holographic informationrecording and/or reproducing apparatus, and more particularly, to aholographic information recording and/or reproducing apparatus using asingle side incidence method for enhancing efficiency of an opticalsystem.

2. Description of the Related Art

Recently, technologies for recording information using a hologram havebecome available on the market. According to an information recordingmethod using the hologram, information is stored in a light sensitiveinorganic crystal or polymer material in an optical interferencepattern. Optical interference fringes are formed by using two laserbeams having coherency. That is, information is recorded by opticalinterference fringes, which are formed by interference between areference light and a signal light having difference paths, causing achemical or physical change on a photosensitive storage medium. In orderto reproduce information from such a recorded interference pattern, areproduction light similar to the reference light used when recording isilluminated onto the interference pattern recorded on the storagemedium. This reproduction light causes diffraction due to theinterference pattern, whereby the signal light is restored and theinformation is reproduced.

In the hologram information recording technology, there is a volumeholography method in which recording and reproducing are performed in apage unit using volume holography, and there is a micro holographymethod in which recording and reproducing are performed in a single bitunit using micro holography. The volume holography method has anadvantage in that bulk information can be processed at the same time.However, since an optical system is minutely controlled, it is difficultfor the volume holography method to be commercialized for an informationstorage apparatus for general users.

The micro holography method records information in a storage medium in athree dimensional method by forming a minute interference pattern bymaking two concentrated beams interfere with each other at a focus andforming an information plane by moving the interference fringes on aplane of the storage medium. The micro holography method layeredlyrecords the information plane in a depth direction of the storagemedium. However, a conventional recording and/or reproducing apparatususing the micro holography method includes an optical system for asignal light and an optical system for a reference light, each lightbeing illuminated onto both sides of a storage medium. Thus, theilluminating of the signal light and the reference light onto both sidesof the storage medium results in complexity for an optical system.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a holographic informationrecording and/or reproducing apparatus to enhance an efficiency ofilluminated light by making signal light and reference light incident ona single side of a holographic information storage medium.

According to an aspect of the present invention, there is provided aholographic information recording and/or reproducing apparatus to recordand/or reproduce information to/from a holographic information storagemedium, the holographic information recording and/or reproducingapparatus including: a first light source unit to emit a reference lightand a signal light in a recording mode, each being linear polarizedlight and orthogonal to each other; a first optical path guiding unit toguide the reference light and the signal light emitted by the firstlight source unit so that the reference light and the signal light crosseach other at a first crossing point after passing through differentoptical paths; a second optical path guiding unit including a firstpolarization converter being located on an optical path of the referencelight or the signal light before the first crossing point, a firstpolarization beam splitter located at the first crossing point of thereference light and the signal light, at least one first optical pathconverter to guide the reference light and/or the signal light so thatthe reference light and the signal light cross again at a secondcrossing point after the first crossing point, a second polarizationconverter located on the optical path of the signal light before thesecond crossing point after passing through the first polarization beamsplitter, and a second polarization beam splitter located at the secondcrossing point to unite the optical paths of the reference light and thesignal light; and an objective lens unit to illuminate the referencelight and the signal light that have passed through the secondpolarization beam splitter onto a single side of a holographicinformation storage medium.

The first polarization converter, the second polarization converter, thefirst polarization beam splitter, the second polarization beam splitter,the at least one first optical path converter, and/or the secondpolarization converter may be combined into one body.

The objective lens unit may include: a quarter-wave plate topolarization-convert the reference light and the signal light to beorthogonally polarized to each other; and an objective lens to cause theinformation to be recorded by interference fringes formed in a depthdirection of the holographic information storage medium around a focusformed by directly focusing the reference light on a focal point of theholographic information storage medium, reflecting the signal light froma reflection layer of the holographic information storage medium withoutpolarization conversion, and focusing the reflected signal light on thefocal point of the reference light.

The objective lens unit may include: a fourth polarization converter topolarization-convert the reference light and the signal light to alinear polarized light in a same polarization direction; a quarter-waveplate to polarization-convert the reference light and the signal lightto a same-directional circular polarized light; and an objective lens tocause the information to be recorded by interference fringes formed in adepth direction of the holographic information storage medium around afocus formed by directly focusing the reference light on a focal pointof the holographic information storage medium, reflecting the signallight from a reflection layer of the holographic information storagemedium with polarization conversion, and focusing the reflected signallight on the focal point of the reference light.

According to another aspect of the present invention, there is provideda holographic information recording and/or reproducing apparatus torecord and/or reproduce information to/from a holographic informationstorage medium, the holographic information recording and/or reproducingapparatus including: a first light source unit to emit a reference lightand a signal light in a recording mode, each being linear polarizedlight and orthogonal to each other; a first optical path guiding unit toguide the reference light and the signal light emitted by the firstlight source unit so that the reference light and the signal light crosseach other at a first crossing point after passing through differentoptical paths; a second optical path guiding unit to guide the referencelight and/or the signal light so that the reference light and the signallight cross again at a second crossing point after passing throughdifferent optical paths after the first crossing point; and an objectivelens unit to illuminate the reference light and the signal light thathave passed through the second polarization beam splitter onto a singleside of the holographic information storage medium.

According to still another aspect of the present invention, there isprovided a holographic information recording and/or reproducingapparatus to record and/or reproduce information to/from a holographicinformation storage medium, the holographic information recording and/orreproducing apparatus including: a first optical path guiding unit toguide a reference light and a signal light, each being linear polarizedlight and orthogonal to each other, so that the reference light and thesignal light cross each other at a first crossing point after passingthrough different optical paths; a second optical path guiding unit toguide the reference light and/or the signal light so that the referencelight and the signal light cross again at a second crossing point afterpassing through different optical paths after the first crossing point;and an objective lens unit to illuminate the reference light and thesignal light that have passed through the second polarization beamsplitter onto a single side of the holographic information storagemedium.

According to another aspect of the present invention, there is provideda method of recording information to a holographic information storagemedium, the method including: emitting a reference light and a signallight, each being linear polarized and orthogonal to each other; guidingthe emitted reference light and the emitted signal light so that thereference light and the signal light cross each other at a firstcrossing point after passing through different optical paths; unitingthe optical paths of the reference light and the signal light; andilluminating the reference light and the signal light onto a single sideof the holographic information storage medium.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a schematic optical configuration of a holographic informationrecording and/or reproducing apparatus according to an embodiment of thepresent invention;

FIG. 2 illustrates a reflective holographic information storage mediumloaded in the holographic information recording and/or reproducingapparatus illustrated in FIG. 1;

FIG. 3 is a schematic optical configuration for illuminating a signallight and a reference light onto the holographic information storagemedium illustrated in FIG. 2 when a numerical aperture for the signallight is the same as that for the reference light;

FIG. 4 is a schematic optical configuration for illuminating a signallight and a reference light onto the holographic information storagemedium illustrated in FIG. 2 when the numerical aperture for the signallight is different from that for the reference light;

FIG. 5 illustrates polarization states of a signal light and a referencelight illuminated onto the holographic information storage mediumillustrated in FIG. 2 in a recording mode;

FIG. 6 illustrates a servo light illuminated onto the holographicinformation storage medium illustrated in FIG. 2 in a servo process;

FIG. 7 illustrates a polarization state of a reproduction lightilluminated onto the holographic information storage medium illustratedin FIG. 2 in a reproduction mode;

FIGS. 8 and 9 illustrate a portion of a second optical path guiding unitbeing formed in one body, which is included in the holographicinformation recording and/or reproducing apparatus illustrated in FIG.1;

FIG. 10 is a schematic optical configuration of a holographicinformation recording and/or reproducing apparatus according to anotherembodiment of the present invention;

FIG. 11 illustrates a reflective holographic information storage mediumequipped in the holographic information recording and/or reproducingapparatus illustrated in FIG. 10;

FIG. 12 illustrates polarization states of a signal light and areference light illuminated onto the holographic information storagemedium illustrated in FIG. 11 in a recording mode;

FIG. 13 illustrates a servo light illuminated onto the holographicinformation storage medium illustrated in FIG. 11 in the servo process;and

FIG. 14 illustrates a polarization state of a reproduction lightilluminated onto the holographic information storage medium illustratedin FIG. 11 by the holographic information recording and/or reproducingapparatus illustrated in FIG. 10.

DETAILED DESCRIPTION OF THE EMBODIMENT

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIG. 1 is a schematic optical configuration of a holographic informationrecording and/or reproducing apparatus according to an embodiment of thepresent invention. Referring to FIG. 1, the holographic informationrecording and/or reproducing apparatus records information on aholographic information storage medium 300 and reproduces recordedinformation. In particular, the holographic information recording and/orreproducing apparatus includes a circuit (not shown) and an opticalsystem including an optical pickup 100 illuminating light onto a singleside of the holographic information storage medium 300 and receiving theilluminated light.

The optical pickup 100 includes a first light source 110, a firstcollimating lens 115, a third polarization converter 120, a thirdpolarization beam splitter 125, first and second focus controlling units130 and 145, first and second mirrors 135 and 140, a first polarizationconverter 150, a first polarization beam splitter 155, a secondpolarization converter 165, a third mirror 170, a wavelength selectivebeam splitter 160, a second polarization beam splitter 175, a fourthmirror 180, a quarter-wave plate 185, an objective lens 190, aconcentration lens 195, and a first optical detector 200. In order toread servo information, the optical pickup 100 further includes a servooptical system including a second light source 210, a diffractiongrating 215, a servo light polarization beam splitter 220, a secondcollimating lens 225, a servo light focus controlling unit 230, adetection lens 235, and a second optical detector 240.

In FIG. 1, a thick solid line indicates a reference light L11 or areproduction light L5 emitted from the first light source 110 to theholographic information storage medium 300, a thick alternate long andshort dash line indicates a signal light L12 emitted from the firstlight source 110 to the holographic information storage medium 300, athick dotted line indicates a reflected signal light L2 or a reflectedreproduction light L6 reflected from the holographic information storagemedium 300 to the first optical detector 200, a thin solid lineindicates a servo light L3 emitted from the second light source 210 tothe holographic information storage medium 300, and a thin dotted lineindicates a reflected servo light L4 reflected from the holographicinformation storage medium 300 to the second optical detector 240.

The first light source 110, the first collimating lens 115, and thethird polarization converter 120 are included in a first light sourceunit emitting the reference light L11 and the signal light L12 in arecording mode and emitting the reproduction light L5 in a reproductionmode. The first light source 110 emits a light L1 for recording andreproduction that has a one-directional linear polarization (forexample, a semiconductor laser diode emitting blue light). The light L1for recording and reproduction is emitted in a modulated state accordingto information to be recorded in the recording mode and emitted in anon-modulated state in the reproduction mode. Hereinafter, it is assumedfor convenience of description that a linear polarization direction ofthe light L1 for recording and reproduction is a P-polarizationdirection.

The first collimating lens 115 regularizes the light L1 for recordingand reproduction to an equilibrium light.

The third polarization converter 120 is an active device (e.g., anactive half-wave plate) of which a polarization conversion function isturned on or off according to a turn-over between the recording mode andthe reproduction mode. The third polarization converter 120polarization-converts the light emitted by the first light source 110 toa light having, for example, polarization components of P-polarizationand S-polarization by acting as a wave plate in the recording mode.Furthermore, the third polarization converter 120 passes the lightemitted by the first light source 110 as is without acting as the waveplate. For this active wave plate, a liquid crystal device that uses abirefringence characteristic of a liquid crystal having an optical axisby being arranged when a voltage is applied may be used. For example,when a voltage is applied to the active half-wave plate, if an anglebetween a linear polarization direction of incident light and a fastaxis of the active half-wave plate is a degree excluding 45°, (such as22.5°), the incident light (e.g., a P-polarized light) is converted tolight having two orthogonal polarization components (i.e., aP-polarization component and an S-polarization component) due to arotation of the polarization direction of the incident light whilepassing through the active half-wave plate. The P-polarization componentand the S-polarization component of which the polarization directionsare rotated correspond to the reference light L11 and the signal lightL12 in the recording mode, respectively. Since a configuration of theactive half-wave plate is well known by one of ordinary skill in theart, a detailed description will be omitted herein. Although the activehalf-wave plate is described as a polarization converter in the currentembodiment, aspects of the present invention are not limited thereof.For example, a drivable wave plate that is located on an optical path inthe recording mode and removed from the optical path in the reproductionmode by a mechanical driving unit may also be used as the polarizationconverter.

The third polarization beam splitter 125, the first and second focuscontrolling units 130 and 145, and the first and second mirrors 135 and140 are included in a first optical path guiding unit to guide thereference light L11 and the signal light L12, which have been emitted bythe first light source unit, so that the reference light L11 and thesignal light L12 cross each other after passing through differentoptical paths.

The third polarization beam splitter 125 determines a transmission or areflection of light according to a polarization direction of the light.For example, the third polarization beam splitter 125 may transmitP-polarized light and reflect S-polarized light. Accordingly, the thirdpolarization beam splitter 125 can split optical paths of the referencelight L11 and the signal light L12 by transmitting the reference lightL11 that is, for example, P-polarized light in the recording mode, andreflecting the signal light L12 that is, for example, S-polarized lightin the reproduction mode. As described later, a portion of the signallight L2 reflected from the holographic information storage medium 300is input to the third polarization beam splitter 125 by passing backwardalong the optical path of the reference light L11 and reflected to thefirst optical detector 200. In addition, as described later, since apolarization direction of the reproduction light L5 illuminated onto theholographic information storage medium 300 is orthogonal to apolarization direction of the reproduction light L6 reflected from theholographic information storage medium 300 in the reproduction mode, thethird polarization beam splitter 125 can separate the reproduction lightL6 reflected from the holographic information storage medium 300 from anoptical path of the reproduction light L5 emitted from the first lightsource 110 to the holographic information storage medium 300.

The first and second mirrors 135 and 140 are examples of an optical pathconverter and cause the split optical paths of the reference light L11and the signal light L12 cross each other.

The first and second focus controlling units 130 and 145 arerespectively located on the optical path of the reference light L11 andthe optical path of the signal light L12. The first focus controllingunit 130 forms a focal point (F of FIG. 3) of the reference light L11 ata different location in a depth direction of the holographic informationstorage medium 300 by changing a focal point of the objective lens 190for the reference light L11. Likewise, the second focus controlling unit145 forms a focal point of the signal light L12 at a different locationin the depth direction of the holographic information storage medium 300by changing a focal point of the objective lens 190 for the signal lightL12. In this case, since the signal light L12 is focused on the sameposition as the focal point F of the reference light L11 after beingreflected from a reflection layer (340 in FIG. 2) in the holographicinformation storage medium 300, a focal distance of the objective lens190 for the signal light L12 is longer than that for the reference lightL11. That is, the first and second focus controlling units 130 and 145can control a numerical aperture and a focal distance of the opticalsystem with the objective lens 190 by controlling a convergence or adivergence of the reference light L11 and the signal light L12. Asdescribed above, when the reference light L11 and the signal light L12are focused on different locations in the depth direction of theholographic information storage medium 300, a plurality of informationplanes in which information is recorded can be formed.

The first focus controlling unit 130 may employ an active relay lensunit. The active relay lens unit includes, for example, a plurality oflenses 131 and 132, wherein at least one lens 131 is movable in anoptical axis direction and driven by a driving unit (not shown).Likewise, the second focus controlling unit 145 may employ an activerelay lens unit including a plurality of lenses 146 and 147.

The first and second polarization converters 150 and 165, the first andsecond polarization beam splitters 155 and 175, the wavelength selectivebeam splitter 160, and the third mirror 170 are included in a secondoptical path guiding unit. The first polarization converter 150polarization-converts an incident light in an orthogonal polarizationdirection and may employ a half-wave plate. The first polarizationconverter 150 may be located on the optical path of the reference lightL11 or the signal light L12 between the third polarization beam splitter125 and the first polarization beam splitter 155. In the currentembodiment, the first polarization converter 150 is located on theoptical path of the signal light L12 between the third polarization beamsplitter 125 and the first polarization beam splitter 155. Since thesignal light L12 split by the third polarization beam splitter 125 hasan S-polarization, the signal light L12 is converted to a P-polarizationby the first polarization converter 150. That is, the reference lightL11 and the signal light L12 incident to the first polarization beamsplitter 155 have the same linear polarization component. When ahalf-wave plate is used as the first polarization converter 150, theconversion between the P-polarized light and the S-polarized light maybe performed by maintaining 45° between a linear polarization directionof the incident light and a fast axis of the half-wave plate.

The first polarization beam splitter 155, for example, transmitsP-polarized light as is and reflects S-polarized light. Accordingly, thefirst polarization beam splitter 155 transmits the reference light L11and the signal light L12 incident from the first optical path guidingunit as they are. However, as described later, the signal light L2reflected from the holographic information storage medium 300 in therecording mode or the reproduction light L6 reflected in thereproduction mode is reflected from the first polarization beam splitter155, and then proceeds backward along the optical path of the signallight L12 to the third polarization beam splitter 125 by passingbackward along the optical path of the reference light L11.

The second polarization converter 165 may employ, for example, an activewave plate (such as an active half-wave plate). Since the secondpolarization converter 165 is similar to the third polarizationconverter 120, a configuration of the second polarization converter 165will not be described in detail herein. The second polarizationconverter 165 acts as a wave plate in the recording mode and transmitslight without polarization conversion in the reproduction mode. Thesecond polarization converter 165 is located on the optical path of thesignal light L12 before meeting the reference light L11 after passingthrough the first polarization beam splitter 155 in the recording modeand converts the polarization of the signal light L12 so that thepolarization of the signal light L12 is orthogonal to the polarizationof the reference light L11.

As described later, in order to detect a portion of the signal light L2reflected from the holographic information storage medium 300, thesecond polarization converter 165 may leave a portion of one linearpolarized light without completely converting the portion to the otherorthogonal linear polarized light in the recording mode. That is, whenS-polarized light is incident in the recording mode, the secondpolarization converter 165 may leave a portion of the S-polarizationcomponent while converting the rest of the light from the S-polarizationcomponent to a P-polarization component. When an active half-wave plateis used as the second polarization converter 165, if an angle between alinear polarization direction of incident light and a fast axis of theactive half-wave plate is, for example, 28.5°, the polarizationdirection of the incident light (e.g., S-polarized light) is rotatedwhen the S-polarized light passes through the active half-wave plate.Thus, the S-polarized light is converted to light having a dominantP-polarization component and a minor S-polarization component.

The wavelength selective beam splitter 160 is a dichroic mirror actingas a simple mirror for a wavelength of the first light source 110 (i.e.,the light L1 for recording and reproduction) and simply transmitting awavelength of the second light source 210 described later (i.e., theservo light L3). The wavelength selective beam splitter 160 can performa function of combining optical paths of the reference light L11 and theservo light L3, which will be described later.

The wavelength selective beam splitter 160 and the third mirror 170refract an optical path so that the reference light L11 and the signallight L12 passing through the first polarization beam splitter 155 bycrossing each other meet each other again.

The second polarization beam splitter 175 acts as a polarization beamsplitter for wavelengths of the reference light L11 and the signal lightL12 and transmits wavelengths of the servo lights L3 and L4. That is,the second polarization beam splitter 175 is a wavelength selectiveoptical device. Thus, optical paths of the reference light L11 and thesignal light L12 met by the second polarization beam splitter 175 arecombined to one and directed to the objective lens 190 and, as describedlater, the servo lights L3 and L4 are transmitted without changing theiroptical paths.

Since the first and second polarization converters 150 and 165, thefirst and second polarization beam splitters 155 and 175, the wavelengthselective beam splitter 160, and the third mirror 170 (which areincluded the second optical path guiding unit) are optical deviceshaving high optical efficiency, the holographic information recordingand/or reproducing apparatus according to aspects of the presentinvention can record and/or reproduce information with high opticalefficiency as compared to a conventional holographic informationrecording and/or reproducing apparatus using simple beam splitters andhalf mirrors.

The fourth mirror 180 refracts an optical path so that the referencelight L11 and the signal light L12 combined by the second polarizationbeam splitter 175 are directed to the objective lens 190.

The quarter-wave plate 185 changes a polarization direction of lightincident to the holographic information storage medium 300 and apolarization direction of light reflected from the holographicinformation storage medium 300 for the servo lights L3 and L4 and thereproduction lights L5 and L6. That is, the quarter-wave plate 185 is adual-wavelength quarter-wave plate acting as a wave plate for both thefirst light source 110 and the second light source 210. The reproductionlight L6 reflected by the quarter-wave plate 185 is separated from thelight L1 for recording and reproduction by the third polarization beamsplitter 125 and can be detected by the first optical detector 200. Thereflected servo light L4 is separated by the servo light polarizationbeam splitter 220 and can be detected by the second optical detector240. However, the signal light L2 reflected from the holographicinformation storage medium 300 in the recording mode has the samepolarization direction as that of the incident signal light L12. Thus,the reflected signal light L2 can be detected by the first opticaldetector 200 by passing through the same optical path as that of thereproduction light L6 reflected by the second optical path guiding unit.

The objective lens 190 is a lens to concentrate the reference light L11and the signal light L12, the reproduction light L5, or the servo lightL3 on a predetermined area of the holographic information storage medium300. The objective lens 190 can change focal points of the referencelight L11 and the signal light L12 on the holographic informationstorage medium 300 with the first and second focus controlling units 130and 145 and may change a numerical aperture of the optical system. Theobjective lens 190 causes the reference light L11 and the signal lightL12 to be incident to the holographic information storage medium 300,whereby the reference light L11 is focused on the focal point (F of FIG.3) in the holographic information storage medium 300 and the signallight L12 is reflected from the reflection layer (340 of FIG. 2) in theholographic information storage medium 300 and focused on the sameposition as the focal point F of the reference light L11. Furthermore,as described later, the servo light L3 is concentrated on a servo layer(320 of FIG. 6) in the holographic information storage medium 300.

The first optical detector 200 detects the signal light L2 or thereproduction light L6 reflected from the holographic information storagemedium 300. The concentration lens 195 to concentrate the reflectedsignal light L2 or the reflected reproduction light L6 may further beincluded between the third polarization beam splitter 125 and the firstoptical detector 200.

The servo optical system will now be described. According to aspects ofthe present invention, the holographic information storage medium usedin the holographic information recording and/or reproducing apparatusincludes the servo layer (320 of FIG. 2), and the optical pickup 100includes the optical system for reading servo information recorded inthe servo layer 320.

The second light source 210 emits the servo light L3 and may employ, forexample, a semiconductor laser diode emitting a red light. The secondlight source 210 may emit one-directional linear polarized light L inorder to split the servo light L3 incident to the holographicinformation storage medium 300 and the servo light L4 reflected from theholographic information storage medium 300 according to the respectivepolarization directions in the servo light polarization beam splitter220. The diffraction grating 215 is an optical member diffracting theservo light L3 emitted by the second light source 210 to 0^(th)-orderdiffraction light and +1^(st)- or −1^(st)- order diffraction light andis used to detect a servo error signal using a push-pull method. Thesecond collimating lens 225 regulates the servo light L3 emitted by thesecond light source 210 to a parallel light. The servo lightpolarization beam splitter 220 may employ, for example, a polarizationbeam splitter that splits the servo light L3 incident to the holographicinformation storage medium 300 and the servo light L4 reflected from theholographic information storage medium 300 according to their respectivepolarization directions. The servo light focus controlling unit 230varies a focal point of the servo light L3 in the holographicinformation storage medium 300 in the depth direction and may employ arelay lens unit including a plurality of lenses 231 and 232, wherein atleast one lens 231 is movably assembled in an optical axis direction anddriven by a driving unit (not shown). The detection lens 235 makes anoptical spot of the reflected servo light L4 properly formed on thesecond optical detector 240 and may employ, for example, an astigmaticlens so that a focus error signal can be detected by astigmatism. Thesecond optical detector 240 includes a plurality of optical detectingunits and detects servo information and a servo error signal included inthe servo layer (320 of FIG. 2) of the holographic information storagemedium 300. The servo optical system described above is an example ofusing servo light having a wavelength different from that of the lightfor recording and reproduction, and it is understood that aspects of thepresent invention are not limited thereto.

FIG. 2 illustrates a reflective holographic information storage medium300 loaded in the holographic information recording and/or reproducingapparatus illustrated in FIG. 1. Referring to FIG. 2, the holographicinformation storage medium 300 used in the holographic informationrecording and/or reproducing apparatus is a reflective storage mediumhaving a structure in which a substrate 310, a servo layer 320, a bufferlayer 330, a reflection layer 340, a space layer 350, a recording layer360, and a cover layer 370 are sequentially layered.

The servo layer 320 is a layer in which servo information is recordedand reflects servo light (L3 of FIG. 1). The buffer layer 330 may beformed with a transparent material or a material absorbing a wavelengthof light for recording and reproduction. Although the servo layer 320 islocated below the reflection layer 340 in FIG. 2, it is understood thataspects of the present invention are not limited thereto. As describedlater with reference to FIG. 9, the servo layer 320 may be above or inthe recording layer 360.

The reflection layer 340 reflects the signal light L1 so that the signallight L1 concentrates on the focal point (F of FIG. 3) in theholographic information storage medium 300. In the current embodiment,the reflection layer 340 employs a circular polarized-light splitreflection layer formed with a material reflecting light having firstcircular polarization and transmitting light having second circularpolarization, wherein the first circular polarization direction isorthogonal to the second circular polarization direction. The circularpolarized-light split reflection layer may be formed with, for example,a cholesteric liquid crystal of a liquid crystal film in a liquidcrystal state or a hardened state. The cholesteric liquid crystal has astructure whereby directors of liquid crystal molecules are twisted in aspiral pattern, and can split the signal light L1 to two orthogonalcircular polarized lights by reflecting circular polarized lightcorresponding the spiral pattern and transmitting circular polarizedlight corresponding an inverse direction to the spiral pattern, whereinthe reflected light is maintained in an original circular polarizationstate. Furthermore, the reflection layer 340 is designed to transmit theservo light L3, and to transmit the reference light L3 in order toreduce noise.

The space layer 350 is a layer to secure a space between the recordinglayer 360 and the reflection layer 340. The space layer 350 may be usedto remove noise due to partial light defocused and reflected by thereflection layer 340. The recording layer 360 is formed with aphotosensitive material (e.g., a photo polymer or a thermoplasticmaterial) of which a refractive index varies when light is absorbed. Thecover layer 370 is a layer to protect the recording layer 360 from theoutside.

A recording and/or reproducing method of the holographic informationrecording and/or reproducing apparatus according to an embodiment of thepresent invention will now be described with reference to FIGS. 3through 7.

The recording mode of the holographic information recording and/orreproducing apparatus will be described first. FIG. 3 is a schematicoptical configuration for illuminating the signal light L12 and thereference light L11 onto the holographic information storage medium 300illustrated in FIG. 2 in the recording mode. Since each layer of theholographic information storage medium 300 illustrated in FIG. 3 issimilar to those illustrated in FIG. 2, a detailed description will beomitted herein.

Referring to FIG. 3, the reference light L11 and the signal light L12respectively having P- and S-polarization components are illuminatedonto the holographic information storage medium 300 by the objectivelens 190 after moving along the same optical path. In this case,convergence and divergence are controlled by the first and secondoptical path guiding units. In particular, the first and second focuscontrolling units 130 and 145 guide the reference light L11 and thesignal light L12, wherein the reference light L11 is concentrated on thefocal point F in the recording layer 360 right after passing through thecover layer 370 and the signal light L12 is concentrated on the focalpoint F in the recording layer 360 after passing through the cover layer370 and the recording layer 360 and being reflected from the reflectionlayer 340.

Accordingly, since optical spots of the reference light L11 and thesignal light L12 meet each other on the focal point F, bulk-typeinterference fringes are formed around the focal point F. Since apattern of these interference fringes varies according to a modulatedstate of the signal light L12 or a modulated state of the referencelight L11 and the signal light L12, information can be recordedaccording to the interference fringes. The interference fringes can forma single information plane 365 in the recording layer 360 by beingrecorded along tracks on the same side, or information can be recordedin a plurality of planes by forming interference fringes while a focalpoint is changed in the depth direction of the recording layer 360. Theholographic information storage medium 300 may use a micro holographymethod in which single-bit information is contained in an interferencepattern for every focal point F, though it is understood that aspects ofthe present invention are not limited thereto. For example, theholographic information storage medium 300 may use a volume holographymethod in which a plurality of pieces of information are simultaneouslyrecorded by stereoscopic interference fringes formed due to overlappingof the optical spots of the reference light L11 and the signal lightL12.

The description of the reference light L11 and the signal light L12 withreference to FIG. 3 is for a case where a numerical aperture for thesignal light L12 is the same as that for the reference light L11.However, it is understood that aspects of the present invention are notlimited thereto. As described above, a numerical aperture of the opticalsystem may be controlled by using the first and second focus controllingunits 130 and 145 and the objective lens 190. Accordingly, a numericalaperture of the objective lens 190 for the reference light L11 may bedifferent from that for the signal light L12 according to other aspects.

FIG. 4 is a schematic optical configuration for illuminating the signallight L12 and the reference light L11 onto the holographic informationstorage medium 300 illustrated in FIG. 2 when the numerical aperture forthe signal light L12 is smaller than that for the reference light L11.Since reference numerals in FIG. 4 are the same as those in FIG. 3, adetailed description of components in FIG. 4 is omitted herein.

Referring to FIG. 4, since a numerical aperture of the objective lens190 for the signal light L12 is relatively smaller, a focal distance ofthe signal light L12 is relatively longer in a case of the same beamwidth. Thus, an optical spot of the signal light L12 is formed on thefocal point F after the signal light L12 is reflected from thereflection layer 340. However, since a numerical aperture of theobjective lens 190 for the reference light L11 is relatively larger, afocal distance of the reference light L11 is relatively shorter in acase of the same beam width. Thus, an optical spot of the referencelight L11 is directly formed on the focal point F.

By forming an optical configuration as described above, an opticalsystem having a smaller numerical aperture for at least the signal lightL12 can be achieved. Thus, a margin can be obtained in a case ofaberration allowed in a design of optical devices or assemblingtolerance in a manufacturing process of optical pickups.

FIG. 5 illustrates polarization states of the signal light L12 and thereference light L11 illuminated onto the holographic information storagemedium illustrated 300 in FIG. 2. Referring to FIG. 5, the referencelight L11 and the signal light L12, each having a different linearpolarization component, are incident to the quarter-wave plate 185. Forexample, the reference light L11 is incident to the quarter-wave plate185 in a P-polarization state, and the signal light L12 is incident tothe quarter-wave plate 185 in an S-polarization state. The quarter-waveplate 185 is an optical member converting linear polarized light tocircular polarized light and circular polarized light to linearpolarized light. In terms of the polarization states, the polarizationof the signal light L12 is converted to a left circular polarization Lafter passing through the quarter-wave plate 185, and the polarizationof the reference light L11 is converted to a right circular polarizationR after passing through the quarter-wave plate 185. The signal light L12that has the left circular polarization L maintains its originalpolarization state by being reflected from the reflection layer 340 asis. The reflected signal light L2 having the left circular polarizationL is focused in the information plane 365. However, the reference lightL11 having the right circular polarization R is directly focused on theinformation plane 365 right after passing through the cover layer 370.Since the reflected signal light L2 and the reference light L11 meetingin the information plane 365 move in opposite directions and haveopposite circular polarization directions, an electric field vector ofthe reflected signal light L2 rotates in the same direction as that ofan electric field vector of the reference light L11, and thus,interference occurs in the information plane 365. This interferencecauses information to be recorded on the recording layer 360 that ismade of a photosensitive material.

The signal light L2 reflected from the reflection layer 340 formsinterference fringes at the focal point F and continues to an outside ofthe holographic information storage medium 300 via the cover layer 370.Since the reflected signal light L2 maintains the left circularpolarization state L, the reflected signal light L2 is converted to anS-polarized light after passing through the quarter-wave plate 185.

Referring back to FIG. 1, the reflected signal light L2 that isS-polarized light is reflected from the second polarization beamsplitter 175 and passes through the second polarization converter 165.Since the second polarization converter 165 is active in the recordingmode, the reflected signal light L2 is polarization converted. Asdescribed above, when S-polarized light is incident to the secondpolarization converter 165 in the recording mode, the secondpolarization converter 165 leaves a portion of an S-polarizationcomponent unconverted while converting the remainder of theS-polarization component to a P-polarization component. The reflectedsignal light L2 having the portion of the S-polarization component isreflected from the first polarization beam splitter 155 and movesbackward along the optical path of the reference light L11. That is, thesignal light L2 having the S-polarization component, which has beenreflected from the first polarization beam splitter 155, is incident tothe third polarization beam splitter 125 after passing through the firstmirror 135 and the first focus controlling unit 130. In the currentembodiment, since the third polarization beam splitter 125 reflectsS-polarized light, the signal light L2 moving backward along the opticalpath of the reference light L11 and being incident to the thirdpolarization beam splitter 125 is reflected from the third polarizationbeam splitter 125 to continue to the first optical detector 200.

As described above, information on the signal light L2 detected by thefirst optical detector 200 in the recording mode may be used for a focusservo so that the reference light L11 and the signal light L12 arefocused on the information plane 365 in the holographic informationstorage medium 300 by controlling the first and second focus controllingunits 130 and 145.

Servo information detection of the holographic information recordingand/or reproducing apparatus according to current embodiment will now bedescribed with reference to FIG. 6. FIG. 6 illustrates a servo lightilluminated onto and reflected from the holographic information storagemedium 300 illustrated in FIG. 2. Since each layer of the holographicinformation storage medium 300 illustrated in FIG. 6 is similar to thoseillustrated in FIG. 2, a detailed description will be omitted herein.

Referring to FIG. 6, one-directional linear polarized light (e.g., theservo light L3 that is P-polarized light) is incident to the holographicinformation storage medium 300 via the quarter-wave plate 185 and theobjective lens 190. The servo light L3 is converted from the P-polarizedlight to the left circular polarized light while passing through thequarter-wave plate 185. The servo light L3 incident to the holographicinformation storage medium 300 is reflected from the servo layer 320. Inthis case, since a rotation direction of a polarization vector of theservo light L3 is not changed but an orientation of light is changed inan opposite direction, the left circular polarized light is converted toa right circular polarized light. The reflected servo light L4 isconverted to an S-polarized light while passing through the quarter-waveplate 185 and moves backward along the optical path of the incidentservo light L3. Referring back to FIG. 1, the reflected servo light L4passes through the second polarization beam splitter 175 and thewavelength selective beam splitter 160 without changing an optical path,is reflected from the servo light polarization beam splitter 220 afterpassing through the servo light focus controlling unit 230 and thesecond collimating lens 225, and is detected by the second opticaldetector 240. Since information regarding tracks is contained in theservo layer 320 of the holographic information storage medium 300,tracking of recording marks recorded with bulk-type interference fringescan be performed by reading servo information contained in the servolayer 320.

The reproduction mode of the holographic information recording and/orreproducing apparatus according will now be described with reference toFIG. 7. FIG. 7 illustrates a polarization state of reproduction lightilluminated onto the holographic information storage medium 300illustrated in FIG. 2.

Referring to FIG. 7, the reproduction light L5 having the samepolarization direction as that of the reference light L11 is illuminatedonto the holographic information storage medium 300 for reproduction. Inthis case, since the third polarization converter 120 does not performthe polarization conversion operation, the reproduction light L5 isguided to the holographic information storage medium 300 with the samepolarization direction as that of the reference light L11 (e.g., aP-polarization direction). A focal point of the reproduction light L5can be formed in the desired information plane 365 of the holographicinformation storage medium 300 by using the first focus controlling unit130 located on the optical path of the reproduction light L5. Thereproduction light L5 that is P-polarized light is converted to a rightcircular polarized light R by the quarter-wave plate 185 and is incidentto the holographic information storage medium 300 via the objective lens190. The reproduction light L5 focused on the information plane 365 isreflected from the information plane 365 with information included ininterference fringes formed on the information plane 365. That is, thereproduction light L5 incident in the right circular polarization stateis diffracted (i.e., reflected) from the information plane 365 on whichinformation is recorded. Since an orientation of the reproduction lightL6 reflected from the information plane 365 is changed but a rotationdirection of an electric field vector is not changed, the reflectedreproduction light L6 becomes in the left circular polarization state.The reflected reproduction light L6 that is left circular polarizedlight L is converted to S-polarized light by the quarter-wave plate 185and moves backward along the optical path of the incident reproductionlight L5. As described above, the reflected reproduction light L6 isreflected from the third polarization beam splitter 125 and detected bythe first optical detector 200.

FIG. 8 is an example obtained by modifying the second optical pathguiding unit according the embodiment described above and illustrates aconfiguration in which optical devices included in an area A of theholographic information recording and/or reproducing apparatusillustrated in FIG. 1 are combined into one body. Referring to FIG. 8,first and second polarization beam splitters 251 and 265 and awavelength selective beam splitter 261 are cubic beam splitters, and athird mirror 255 may include a cubic prism. The first polarization beamsplitter 251 and the third mirror 255 are included in a first opticaldevice 250 by being combined into one body. Similarly, the secondpolarization beam splitter 265 and the wavelength selective beamsplitter 261 are included in a second optical device 260 by beingcombined into one body. The second polarization converter 165 may belocated between the third mirror 255 and the second polarization beamsplitter 265.

FIG. 9 is another example of the second optical path guiding unitaccording to the embodiment described above and illustrates an opticaldevice 270 obtained by combining the optical devices included in thearea A of the holographic information recording and/or reproducingapparatus illustrated in FIG. 1 into one body. First and secondpolarization beam splitters 271 and 281 and a wavelength selective beamsplitter 279 are cubic beam splitters, and a third mirror 273 mayinclude a cubic prism. A second polarization converter 275 is an activehalf-wave plate and may be located between the third mirror 273 and thesecond polarization beam splitter 281. By locating a transparent plate277 having a same thickness as that of the second polarization converter275 between the first polarization beam splitter 271 and the wavelengthselective beam splitter 279, a thickness difference due to the secondpolarization converter 275 between the third mirror 273 and the secondpolarization beam splitter 281 can be compensated for.

The modifications described with reference to FIGS. 8 and 9 are onlyexamples, and other various modifications can be provided according toother aspects of the present invention. For example, referring back toFIG. 1, the first polarization beam splitter 155 and the wavelengthselective beam splitter 160 may be combined into one body, and/or thethird mirror 170 and the second polarization beam splitter 175 may becombined into one body. Such an optical configuration whereby componentsare combined into one body may reduce an installation space of opticalparts. In FIGS. 8 and 9, optical paths of the reference light L11, thesignal lights L12 and L2, the servo lights L3 and L4, and thereproduction lights L5 and L6 are the same as those described withreference to FIG. 1.

FIG. 10 is a schematic optical configuration of a holographicinformation recording and/or reproducing apparatus according to anotherembodiment of the present invention. Referring to FIG. 10, theholographic information recording and/or reproducing apparatus recordsinformation on a holographic information storage medium 600 andreproduces recorded information. Furthermore, the holographic recordingand/or reproducing apparatus includes a circuit (not shown) and anoptical pickup 400 illuminating light onto a single side of theholographic information storage medium 600 and receiving the illuminatedlight. The optical pickup 400 includes a first light source 410, afourth polarization beam splitter 415, a first collimating lens 420, athird polarization converter 425, a third polarization beam splitter430, first and second focus controlling units 435 and 450, first andsecond mirrors 440 and 445, first and second polarization converters 455and 470, first and second polarization beam splitters 460 and 480, awavelength selective beam splitter 465, a third mirror 475, a fourthpolarization converter 485, a fourth mirror 490, a quarter-wave plate495, an objective lens 500, first and second optical detectors 510 and520, a concentration lens 505, and a first detection lens 515. In orderto read servo information, the optical pickup 400 may further include aservo optical system including a second light source 530, a diffractiongrating 535, a servo light polarization beam splitter 540, a secondcollimating lens 545, a servo light focus controlling unit 550, a seconddetection lens 555, and a third optical detector 560.

In FIG. 10, a thick solid line indicates a reference light L11 or areproduction light L5 emitted from the first light source 410 to theholographic information storage medium 600, a thick alternate long andshort dash line indicates a signal light L12 emitted from the firstlight source 410 to the holographic information storage medium 600, athick dotted line indicates a reflected signal light L2 reflected fromthe holographic information storage medium 600 to the second opticaldetector 520, a thick alternate long-short-short dash line indicates areflected reproduction light L6 reflected from the holographicinformation storage medium 600 to the first optical detector 510, a thinsolid line indicates a servo light L3 emitted from the second lightsource 530 to the holographic information storage medium 600, and a thindotted line indicates a reflected servo light L4 reflected from theholographic information storage medium 600 to the third optical detector560.

Optical members according to the current embodiment, which are similarto those of the holographic information recording and/or reproducingapparatus described with reference to FIG. 1, will be described inlesser detail than above with reference to FIG. 1.

The first light source 410, the fourth polarization beam splitter 415,the first collimating lens 420, and the third polarization converter 425are included in a first light source unit emitting the reference lightL11 and the signal light L12 in the recording mode and emitting thereproduction light L5 in the reproduction mode.

The first light source 410 emits a light L1 for recording andreproduction that has a modulated one-directional linear polarization inthe recording mode and a non-modulated one-directional linearpolarization in the reproduction mode according to information to berecorded. For example, the first light source 410 may employ asemiconductor laser diode emitting a blue light.

The fourth polarization beam splitter 415 transmits the light L1 havingthe one-directional linear polarization as is and reflects light havinga linear polarization orthogonal to the polarization of the light L1. Asdescribed later, since the polarization of the signal light L2 reflectedfrom the holographic information storage medium 600 in the recordingmode is orthogonal to the polarization of the light L1 for recording andreproduction on the same optical path, the reflected signal light L2 isreflected to the second optical detector 520. For example, when thefirst light source 410 emits the light L1 for recording and reproductionthat is P-polarized light, the signal light L2 reflected from theholographic information storage medium 600 has an S-polarization. Thus,the reflected signal light L2 is reflected from the fourth polarizationbeam splitter 415 to the second optical detector 520.

An optical emitting surface of the third polarization converter 425 isdivided into a first polarization conversion area 425 a in a centerthereof and a first transparent area 425 b surrounding the center. Thefirst polarization conversion area 425 a polarization-convertsP-polarized light emitted by the first light source 410 to S-polarizedlight in the recording mode, and passes the P-polarized light emitted bythe first light source 410 without polarization conversion in thereproduction mode. The first transparent area 425 b passes theP-polarized light emitted by the first light source 410 withoutpolarization conversion regardless of the recording mode or thereproduction mode. Thus, the light L1 for recording and reproduction isdivided into the signal light L12 polarization-converted by passingthrough the polarization conversion area 425 a and the reference lightL11 passing through the first transparent area 425 b withoutpolarization conversion when the light L1 for recording and reproductionpasses through the third polarization converter 425. Although thereference light L11 appears to pass through the polarization conversionarea 425 a in FIG. 10, this is only for convenience of drawing and doesnot indicate that the reference light L11 passes through thepolarization conversion area 425 a.

Although the polarization conversion area 425 a is in the center and thefirst transparent area 425 b is in the surrounding area in the currentembodiment, it is understood that aspects of the present invention arenot limited thereto. That is, according to other aspects, thepolarization conversion area 425 a is in the surrounding area and thefirst transparent area 425 b is in the center.

The third polarization beam splitter 430, the first and second focuscontrolling units 435 and 450, and the first and second mirrors 440 and445 are included in a first optical path guiding unit guiding thereference light L11 and the signal light L12 so that the reference lightL11 and the signal light L12 cross each other after passing throughdifferent optical paths. Since the optical devices included in the firstoptical path guiding unit are similar to those described with referenceto FIG. 1, they will not be described herein. However, unlike thatdescribed with reference to FIG. 1, the first optical detector 510 doesnot detect the reflected signal light L2 because although the reflectedsignal light L2 is directed from the first polarization beam splitter460 to the third polarization beam splitter 430 by moving backward alongan optical path of the reference light L11, the reflected signal lightL2 penetrates the third polarization beam splitter 430 since apolarization direction of the reflected signal light L2 is the same asthat of the reference light L11. Polarization of the reflected signallight L2 will be described later.

The first and second polarization converters 455 and 470, the first andsecond polarization beam splitters 460 and 480, the wavelength selectivebeam splitter 465, and the third mirror 475 are included in a secondoptical path guiding unit. These optical devices included the secondoptical path guiding unit are similar to those described with referenceto FIG. 1. Furthermore, like modified examples described with referenceto FIGS. 8 and 9, optical parts included in the second optical pathguiding unit may be combined into one body. For example, the secondpolarization converter 470, the first and second polarization beamsplitters 460 and 480, the wavelength selective beam splitter 465,and/or the third mirror 475 may be combined into one body as a singleoptical part (such as the modified example described with reference toFIG. 9).

An optical path of the reflected signal light L2 or reflectedreproduction light L6 is different from that described with reference toFIG. 1, and this will be described later.

The fourth polarization converter 485 is located between the secondpolarization beam splitter 480 and the fourth mirror 490. The fourthpolarization converter 485 corresponds to the third polarizationconverter 425, and an optical emitting surface of the fourthpolarization converter 485 is divided into a second polarizationconversion area 485 a in the center and a second transparent area 485 bsurrounding the center. The second polarization conversion area 485 apolarization-converts P-polarized light to S-polarized light andS-polarized light to P-polarized light in the recording mode and passeslight without polarization conversion in the reproduction mode. Sincethe signal light L12 passing through the center of a light flux or thereference light L11 passing through the surroundings of the light fluxmaintains a spatial distribution even if the light passes through thefirst and second optical path guiding units, the signal light L12 passesthrough the second polarization conversion area 485 a, and the referencelight L11 passes through the second transparent area 485 b. In addition,the signal light L12 passing through the second polarization conversionarea 485 a is polarization-converted, and the reference light L11passing through the second transparent area 485 b is notpolarization-converted.

As described above, the reference light L11 and the signal light L12respectively corresponding to an extension light flux and an intensionlight flux are illuminated onto the holographic information storagemedium 600. Although the current embodiment provides the fourthpolarization converter 485 corresponding to the third polarizationconverter 425 such that the second polarization conversion area 485 atakes the center area and the second transparent area 485 b takes thesurrounding area, it is understood that aspects of the present inventionare not limited thereto.

The first optical detector 510 detects the reproduction light L6reflected from the holographic information storage medium 300. Theconcentration lens 505 to concentrate the reflected reproduction lightL6 may further be included between the third polarization beam splitter430 and the first optical detector 510.

The second optical detector 520 detects the signal light L2 reflectedfrom the holographic information storage medium 300. The first detectionlens 515 to properly concentrate an optical spot of the reflected signallight L2 on the second optical detector 520 may further be includedbetween the fourth polarization beam splitter 415 and the second opticaldetector 520.

Since each component of the servo optical system is similar to thatdescribed with reference to FIG. 1, the servo optical system will not bedescribed herein.

FIG. 11 illustrates a reflective holographic information storage mediumloaded in the holographic information recording and/or reproducingapparatus illustrated in FIG. 10. Referring to FIG. 11, the holographicinformation storage medium 600 is a reflective storage medium and has astructure in which a substrate 610, a reflection layer 620, a spacelayer 630, a recording layer 640, a servo layer 650, and a cover layer660 are sequentially layered.

The holographic information storage medium 600 according to the currentembodiment has different characteristics for the reflection layer 620and a different position of the servo layer 650 from the holographicinformation storage medium 300 described with reference to FIG. 2. Aposition of the servo layer 650 is not limited to a position above thereflection layer 620 as described with reference to FIG. 2, and may belocated at another position according to other aspects.

The reflection layer 620 includes a general reflection film material.Furthermore, unlike the reflection layer 340 of the holographicinformation storage medium 300 illustrated in FIG. 2, when incidentlight as circular polarized light is reflected from the reflection layer620, a polarization direction of the circular polarized light ischanged. Polarization states in the recording mode, the servo process,and the reproduction mode will be described with reference to FIGS. 12through 14.

Polarization states of the signal light L12 and the reference light L11incident to the holographic information storage medium 600 will now bedescribed with reference to FIG. 12. Referring to FIG. 12, the referencelight L11 and the signal light L12 having the same linear polarizationare incident to the quarter-wave plate 495. For example, the referencelight L11 and the signal light L12 are incident to the quarter-waveplate 495 in a P-polarization state. The polarization state of thereference light L11 and the signal light L12 are changed to a rightcircular polarization R when the reference light L11 and the signallight L12 pass through the quarter-wave plate 495.

When the signal light L12 that has the right circular polarization R isreflected from the reflection layer 620, the signal light L12 isconverted to a left circular polarized light L. The reflected signallight L2 that has the left circular polarized light L is focused on aninformation plane 665. However, the reference light L11 that has theright circular polarized light R is directly focused on the informationplane 665 right after passing through the cover layer 660. Since thereflected signal light L2 and the reference light L11 meeting in theinformation plane 665 move opposite directions and have oppositecircular polarization directions, interference occurs in the informationplane 665. This interference causes information to be recorded on therecording layer 640 that includes a photosensitive material.

The signal light L2 reflected from the reflection layer 620 formsinterference fringes at the focal point F and continues to an outside ofthe holographic information storage medium 600 via the cover layer 660.Since the reflected signal light L2 maintains the left circularpolarization state L, the reflected signal light L2 is converted to anS-polarized light after passing through the quarter-wave plate 495.

Referring back to FIG. 10, the reflected signal light L2 that isS-polarized passes through the fourth polarization converter 485. Sincethe reflected signal light L2 located in the center area of the lightflux maintains its state even if reflected, the reflected signal lightL2 is polarization-converted to a P-polarized light after passingthrough the second polarization conversion area 485 a of the fourthpolarization converter 485. Thus, the reflected signal light L2 passesthrough the second polarization beam splitter 480 as is and movesbackward along the optical path of the reference light L11. That is, thereflected signal light L2 is incident to the fourth polarization beamsplitter 415 after passing through the second polarization conversionarea 485 a of the fourth polarization converter 485, the secondpolarization beam splitter 480, the first mirror 400, the first focuscontrolling unit 435, the third polarization beam splitter 430, thefirst polarization conversion area 425 a of third polarization converter425, and the first collimating lens 420. In this case, since thereflected signal light L2 is polarization-converted to S-polarized lightby the first polarization conversion area 425 a of the thirdpolarization converter 425, the reflected signal light L2 is reflectedfrom the fourth polarization beam splitter 415 and directed to thesecond optical detector 520. As described above, information on thereflected signal light L2 detected by the second optical detector 520 inthe recording mode may be used for a focus servo so that the referencelight L11 and the signal light L12 are focused on the information plane665 in the holographic information storage medium 600 by controlling thefirst and second focus controlling units 435 and 450.

Servo information detection of the holographic information recordingand/or reproducing apparatus according to the current embodiment willnow be described with reference to FIG. 13. FIG. 13 illustrates servolight L3 and L4 incident to and reflected from the holographicinformation storage medium 600 illustrated in FIG. 11. In FIG. 13, sinceeach layer of the holographic information storage medium 600 is similarto those illustrated in FIG. 11, the layers will not be describedherein.

Referring to FIG. 13, one-directional polarized light (e.g., the servolight L3 that is P-polarized) is incident to the holographic informationstorage medium 600 after passing through the quarter-wave plate 495 andthe objective lens 500. The servo light L3 is converted from theP-polarized light to a left circular polarized light by the quarter-waveplate 495. The servo light L3 incident to the holographic informationstorage medium 600 is reflected from the servo layer 650. In this case,since a rotation direction of a polarization vector of the servo lightL3 is not changed but a light moving direction is opposite to anoriginal direction, the left circular polarized light is converted to aright circular polarized light. The reflected servo light L4 isconverted to an S-polarized light by the quarter-wave plate 495 andmoves backward along an optical path of the servo light L3. Referringback to FIG. 10, the reflected servo light L4 passes through the secondpolarization beam splitter 480 and the wavelength selective beamsplitter 465 without a path change, passes through the servo light focuscontrolling unit 550 and the second collimating lens 545, is reflectedfrom servo light polarization beam splitter 540, and is detected by thesecond optical detector 560. The detected servo information is used bythe optical pickup 400 to perform tracking in the recording orreproduction mode.

The reproduction mode of the holographic information recording and/orreproducing apparatus according to the current embodiment will now bedescribed with reference to FIG. 14. FIG. 14 illustrates a polarizationstate of the reproduction light L5 and L6 incident to the holographicinformation storage medium 600 illustrated in FIG. 11.

Referring to FIG. 14, for reproduction, reproduction light L5 having thesame polarization direction as that of the reference light L11 isilluminated to the holographic information storage medium 600. In thiscase, since the third and fourth polarization converters 425 and 485 donot perform the polarization conversion operation, the reproductionlight L5 is guided to the holographic information storage medium 600with the same polarization direction as that of the reference light L11(e.g., a P-polarization direction). The P-polarization of thereproduction light L5 is converted to a right circular polarization R bythe quarter-wave plate 495 and is incident to the holographicinformation storage medium 600 via the objective lens 500. Thereproduction light L5 focused on the information plane 665 is reflectedfrom the information plane 665 with information on interference fringesformed on the information plane 665. That is, the reproduction light L5incident in the right circular polarization state is diffracted (i.e.,reflected) due to the interference fringes from the information plane665, in which information is recorded, and directed to the objectivelens 500. Since only an orientation of the reproduction light L6reflected from the information plane 665 is changed but a rotationdirection of an electric field vector is not changed, the reflectedreproduction light L6 becomes in the left circular polarization state.The reflected reproduction light L6 having the left circularpolarization L is converted to an S-polarized light by the quarter-waveplate 495, is reflected from the second polarization beam splitter 480,passes through the third mirror 475, the second polarization converter470, the first polarization beam splitter 460, the first mirror 440, andthe first focus controlling unit 435, and is directed to the thirdpolarization beam splitter 430. As described above, the reproductionlight L6 reflected from the holographic information storage medium 600is reflected from the third polarization beam splitter 430 and detectedby the first optical detector 510.

The holographic information recording and/or reproducing apparatusaccording to aspects of the present invention has been described withreference to the embodiments described above. In the embodimentsdescribed above, although a reference light and a reproduction lighthave a P-polarized light and a signal light has an S-polarized light, itis understood that aspects of the present invention are not limitedthereto, and the polarization states may be opposite according to otheraspects. In addition, in the embodiments described above, although theholographic information recording and/or reproducing apparatus performsboth recording and reproduction, aspects of the present invention may beapplied for recording only or reproduction only. Furthermore, in theembodiments described above, although a wavelength of a servo light isdifferent from that of light for recording or reproduction, aspects ofthe present invention are not limited thereto. For example, otheraspects of the present invention may apply even if servo information anda servo error signal are extracted from the light for recording orreproduction.

As described above, according to aspects of the present invention, bymaking a signal light and a reference light incident on a single side ofa holographic information storage medium, the complexity of an opticalsystem can be reduced, and efficiency of the optical system can beincreased.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A holographic information recording and/or reproducing apparatus torecord and/or reproduce information to/from a holographic informationstorage medium, the holographic information recording and/or reproducingapparatus comprising: a first light source unit to emit a referencelight and a signal light in a recording mode, each being linearpolarized light and orthogonal to each other; a first optical pathguiding unit to guide the reference light and the signal light emittedby the first light source unit so that the reference light and thesignal light cross each other at a first crossing point after passingthrough different optical paths; a second optical path guiding unitcomprising a first polarization converter located on an optical path ofthe reference light or the signal light before the first crossing point,a first polarization beam splitter located at the first crossing pointof the reference light and the signal light, at least one first opticalpath converter to guide the reference light and/or the signal light sothat the reference light and the signal light cross again at a secondcrossing point after the first crossing point, a second polarizationconverter located on the optical path of the signal light before thesecond crossing point after passing through the first polarization beamsplitter, and a second polarization beam splitter located at the secondcrossing point to unite the optical paths of the reference light and thesignal light; and an objective lens unit to illuminate the referencelight and the signal light that have passed through the secondpolarization beam splitter onto a single side of the holographicinformation storage medium.
 2. The holographic information recordingand/or reproducing apparatus as claimed in claim 1, wherein the firstpolarization converter is located on the optical path of the signallight before the first crossing point and converts a polarizationdirection of the signal light to an orthogonal direction.
 3. Theholographic information recording and/or reproducing apparatus asclaimed in claim 1, wherein the first polarization converter is ahalf-wave plate.
 4. The holographic information recording and/orreproducing apparatus as claimed in claim 1, wherein the secondpolarization converter converts a polarization direction of the signallight to an orthogonal direction in the recording mode and passes areproduction light without polarization conversion in a reproductionmode.
 5. The holographic information recording and/or reproducingapparatus as claimed in claim 1, wherein the second polarizationconverter is an active half-wave plate.
 6. The holographic informationrecording and/or reproducing apparatus as claimed in claim 1, whereinthe first polarization converter, the second polarization converter, thefirst polarization beam splitter, the second polarization beam splitter,the at least one first optical path converter, and/or the secondpolarization converter are combined into one body.
 7. The holographicinformation recording and/or reproducing apparatus as claimed in claim6, wherein: the first polarization beam splitter and the secondpolarization beam splitter are cubic polarization beam splitters; andthe at least one first optical path converter comprises a first cubicprism converting the optical path of the signal light and a second cubicprism converting the optical path of the reference light.
 8. Theholographic information recording and/or reproducing apparatus asclaimed in claim 7, wherein: the first polarization beam splitter andthe first cubic prism are combined into one body; the secondpolarization beam splitter and the second cubic prism are combined intoone body; and the second polarization converter is located between thefirst cubic prism and the second polarization beam splitter.
 9. Theholographic information recording and/or reproducing apparatus asclaimed in claim 8, wherein: the second polarization converter is anactive half-wave plate; the second optical path guiding unit furthercomprises a transparent plate having a same thickness as that of thesecond polarization converter and located between the first polarizationbeam splitter and the second cubic prism; and the first and the secondpolarization beam splitters, the first and the second cubic prisms, thesecond polarization converter, and the transparent plate are combinedinto one body.
 10. The holographic information recording and/orreproducing apparatus as claimed in claim 1, further comprising: a firstfocus controlling unit located on the optical path of the referencelight between the first light source unit and the objective lens unit tocontrol a focal depth of the reference light illuminated onto theholographic information storage medium; and a second focus controllingunit located on the optical path of the signal light between the firstlight source unit and the objective lens unit to control a focal depthof the signal light illuminated onto the holographic information storagemedium.
 11. The holographic information recording and/or reproducingapparatus as claimed in claim 10, wherein the first and the second focuscontrolling units are active relay lens units in which at least one lensis driven in an optical axis direction.
 12. The holographic informationrecording and/or reproducing apparatus as claimed in claim 1, furthercomprising a servo optical system to read servo information recorded onthe holographic information storage medium.
 13. The holographicinformation recording and/or reproducing apparatus as claimed in claim12, wherein the servo optical system comprises: a second light sourceunit to emit a servo light that is a linear polarized light and has adifferent wavelength from that of the first light source; a servo lightpolarization beam splitter to split the servo light emitted by thesecond light source unit and the servo light reflected from theholographic information storage medium to different optical paths; and aservo optical detector to detect the servo light reflected from theholographic information storage medium and split by the servo lightpolarization beam splitter.
 14. The holographic information recordingand/or reproducing apparatus as claimed in claim 13, wherein: the atleast one first optical path converter comprises a wavelength selectivebeam splitter to combine the optical path of the reference light or thesignal light after the first polarization beam splitter, and an opticalpath of the servo light emitted by the second light source unit; and thesecond polarization beam splitter has a wavelength selectivity such thatthe servo light is transmitted or reflected.
 15. The holographicinformation recording and/or reproducing apparatus as claimed in claim13, wherein the servo optical system further comprises a servo lightfocus controlling unit to control a focal depth of the servo light inthe holographic information storage medium.
 16. The holographicinformation recording and/or reproducing apparatus as claimed in claim1, wherein a numerical aperture of the objective lens unit is a samevalue for the signal light and the reference light.
 17. The holographicinformation recording and/or reproducing apparatus as claimed in claim1, wherein a numerical aperture of the objective lens unit for thesignal light is smaller than a numerical aperture of the objective lensunit for the reference light.
 18. The holographic information recordingand/or reproducing apparatus as claimed in claim 1, wherein theinformation is recorded on a focus as a single bit.
 19. The holographicinformation recording and/or reproducing apparatus as claimed in claim1, wherein the objective lens unit comprises: a quarter-wave plate topolarization-convert the reference light and the signal light to beorthogonally polarized to each other; and an objective lens to cause theinformation to be recorded by interference fringes formed in a depthdirection of the holographic information storage medium around a focusformed by directly focusing the reference light on a focal point of theholographic information storage medium, reflecting the signal light froma reflection layer of the holographic information storage medium withoutpolarization conversion, and focusing the reflected signal light on thefocal point of the reference light.
 20. The holographic informationrecording and/or reproducing apparatus as claimed in claim 19, whereinthe first light source unit comprises: a first light source to emit alight; and a third polarization converter to polarization-convert theemitted light according to the recording or a reproduction mode.
 21. Theholographic information recording and/or reproducing apparatus asclaimed in claim 20, wherein the third polarization converterpolarization-converts the emitted light to have two linear polarizationcomponents orthogonal to each other in the recording mode andpolarization-converts the emitted light to be in a same polarizationdirection as that of the reference light in the reproduction mode. 22.The holographic information recording and/or reproducing apparatus asclaimed in claim 20, wherein: the first light source emits the lighthaving a same polarization direction as that of the reference light; andthe third polarization converter polarization-converts the emitted lightto have two orthogonal linear polarization components in the recordingmode and passes the incident light without polarization conversion inthe reproduction mode.
 23. The holographic information recording and/orreproducing apparatus as claimed in claim 22, wherein the thirdpolarization converter is an active half-wave plate.
 24. The holographicinformation recording and/or reproducing apparatus as claimed in claim19, wherein the first optical path guiding unit comprises: a thirdpolarization beam splitter to split the signal light and the referencelight; and at least one second optical path converter to guide the splitsignal light and the reference light to cross each other.
 25. Theholographic information recording and/or reproducing apparatus asclaimed in claim 19, further comprising a first optical detector todetect a reproduction light reflected from the holographic informationstorage medium in a reproduction mode.
 26. The holographic informationrecording and/or reproducing apparatus as claimed in claim 25, whereinthe first optical path guiding unit comprises: a third polarization beamsplitter to split the signal light and the reference light in therecording mode and to reflect the reproduction light reflected from theholographic information storage medium to the first optical detectoralong the optical path of the reference light in the reproduction mode;and at least one second optical path converter to guide the split signallight and the reference light to cross each other in the recording mode,and to guide the reproduction light reflected from the holographicinformation storage medium to move backward along the optical path ofthe reference light to the third polarization beam splitter in thereproduction mode.
 27. The holographic information recording and/orreproducing apparatus as claimed in claim 25, wherein the secondpolarization converter polarization-converts the signal light reflectedfrom the holographic information storage medium in the recording mode sothat the reflected signal light comprises a portion having apolarization component of the reference light on the same optical pathand causes the reflected signal light to move backward along the opticalpath of the reference light to be detected by the first opticaldetector.
 28. The holographic information recording and/or reproducingapparatus as claimed in claim 1, wherein the objective lens unitcomprises: a fourth polarization converter to polarization-convert thereference light and the signal light to a linear polarized light in asame polarization direction; a quarter-wave plate topolarization-convert the reference light and the signal light to a samedirectional circular polarized light; and an objective lens to cause theinformation to be recorded by interference fringes formed in a depthdirection of the holographic information storage medium around a focusformed by directly focusing the reference light on a focal point of theholographic information storage medium, reflecting the signal light froma reflection layer of the holographic information storage medium withpolarization conversion, and focusing the reflected signal light on thefocal point of the reference light.
 29. The holographic informationrecording and/or reproducing apparatus as claimed in claim 28, whereinthe first light source unit comprises: a first light source to emit alight; and a third polarization converter to polarization-convert theemitted light according to the recording mode or a reproduction mode.30. The holographic information recording and/or reproducing apparatusas claimed in claim 28, wherein the third polarization converterpolarization-converts the emitted light to have two linear polarizationcomponents orthogonal to each other in the recording mode andpolarization-converts the emitted light to have a same polarization asthat of the reference light in the reproduction mode.
 31. Theholographic information recording and/or reproducing apparatus asclaimed in claim 29, wherein: the first light source emits the lighthaving a same polarization as that of the reference light; and the thirdpolarization converter polarization-converts the emitted light to havetwo linear polarization components orthogonal to each other in therecording mode and passes the emitted light without polarizationconversion in the reproduction mode.
 32. The holographic informationrecording and/or reproducing apparatus as claimed in claim 31, wherein:the third polarization converter comprises a first transparent area toalways pass the emitted light without polarization conversion regardlessof the recording mode or the reproduction mode and a first polarizationconversion area to polarization-convert the emitted light in therecording mode, and the fourth polarization converter comprises a secondtransparent area and a second polarization conversion area correspondingto the first transparent area and the first polarization conversionarea, respectively, the second transparent area always passing incidentlight without polarization conversion regardless of the recording modeor the reproduction mode, and the second polarization conversion areapolarization-converting the emitted light in the recording mode.
 33. Theholographic information recording and/or reproducing apparatus asclaimed in claim 29, further comprising: a fourth polarization beamsplitter located between the first light source and the thirdpolarization converter; and a second optical detector located in a sideof the fourth polarization beam splitter, wherein the signal lightreflected from the holographic information storage medium ispolarization-converted in a same direction as that of polarized light ofthe reference light on a same optical path by the fourth polarizationconverter, moves backward along the optical path of the reference light,is polarization-converted by the third polarization converter, is splitby the fourth polarization beam splitter, and is detected by the secondoptical detector.
 34. The holographic information recording and/orreproducing apparatus as claimed in claim 28, further comprising a firstoptical detector to detect a reproduction light reflected from theholographic information storage medium in the reproduction mode.
 35. Theholographic information recording and/or reproducing apparatus asclaimed in claim 34, wherein the first optical path guiding unitcomprises: a third polarization beam splitter to split the signal lightand the reference light in the recording mode and to reflect thereproduction light reflected from the holographic information storagemedium to the first optical detector along the optical path of thereference light in the reproduction mode; and at least one secondoptical path converter to guide the split signal light and the referencelight to cross each other in the recording mode and to guide thereproduction light reflected from the holographic information storagemedium to move backward along the optical path of the reference light tothe third polarization beam splitter in the reproduction mode.
 36. Aholographic information recording and/or reproducing apparatus to recordand/or reproduce information to/from a holographic information storagemedium, the holographic information recording and/or reproducingapparatus comprising: a first light source unit to emit a referencelight and a signal light in a recording mode, each being linearpolarized light and orthogonal to each other; a first optical pathguiding unit to guide the reference light and the signal light emittedby the first light source unit so that the reference light and thesignal light cross each other at a first crossing point after passingthrough different optical paths; a second optical path guiding unit toguide the reference light and/or the signal light so that the referencelight and the signal light cross again at a second crossing point afterpassing through different optical paths after the first crossing point;and an objective lens unit to illuminate the reference light and thesignal light that have passed through the second polarization beamsplitter onto a single side of the holographic information storagemedium.
 37. The holographic information recording and/or reproducingapparatus as claimed in claim 36, wherein the second optical pathguiding unit comprises a first polarization converter located on theoptical path of the signal light before the first crossing point toconvert a polarization direction of the signal light to an orthogonaldirection.
 38. The holographic information recording and/or reproducingapparatus as claimed in claim 37, wherein the second optical pathguiding unit comprises a second polarization converter to convert thepolarization direction of the signal light to an orthogonal direction inthe recording mode and to pass a reproduction light without polarizationconversion in a reproduction mode.
 39. The holographic informationrecording and/or reproducing apparatus as claimed in claim 36, furthercomprising: a first focus controlling unit located on the optical pathof the reference light between the first light source unit and theobjective lens unit to control a focal depth of the reference lightilluminated onto the holographic information storage medium; and asecond focus controlling unit located on the optical path of the signallight between the first light source unit and the objective lens unit tocontrol a focal depth of the signal light illuminated onto theholographic information storage medium.
 40. The holographic informationrecording and/or reproducing apparatus as claimed in claim 36, furthercomprising a servo optical system to read servo information recorded onthe holographic information storage medium.
 41. The holographicinformation recording and/or reproducing apparatus as claimed in claim40, wherein the servo optical system comprises: a second light sourceunit to emit a servo light that is a linear polarized light and has adifferent wavelength from that of the first light source; a servo lightpolarization beam splitter to split the servo light emitted by thesecond light source unit and the servo light reflected from theholographic information storage medium to different optical paths; and aservo optical detector to detect the servo light reflected from theholographic information storage medium and split by the servo lightpolarization beam splitter.
 42. The holographic information recordingand/or reproducing apparatus as claimed in claim 36, wherein theobjective lens unit comprises: a quarter-wave plate topolarization-convert the reference light and the signal light to beorthogonally polarized to each other; and an objective lens to cause theinformation to be recorded by interference fringes formed in a depthdirection of the holographic information storage medium around a focusformed by directly focusing the reference light on a focal point of theholographic information storage medium, reflecting the signal light froma reflection layer of the holographic information storage medium withoutpolarization conversion, and focusing the reflected signal light on thefocal point of the reference light.
 43. The holographic informationrecording and/or reproducing apparatus as claimed in claim 42, whereinthe first light source unit comprises: a first light source to emit alight; and a third polarization converter to polarization-convert theemitted light according to the recording or a reproduction mode.
 44. Theholographic information recording and/or reproducing apparatus asclaimed in claim 42, further comprising a first optical detector todetect a reproduction light reflected from the holographic informationstorage medium in a reproduction mode.
 45. A holographic informationrecording and/or reproducing apparatus to record and/or reproduceinformation to/from a holographic information storage medium, theholographic information recording and/or reproducing apparatuscomprising: a first optical path guiding unit to guide a reference lightand a signal light, each being linear polarized light and orthogonal toeach other, so that the reference light and the signal light cross eachother at a first crossing point after passing through different opticalpaths; a second optical path guiding unit to guide the reference lightand/or the signal light so that the reference light and the signal lightcross again at a second crossing point after passing through differentoptical paths after the first crossing point; and an objective lens unitto illuminate the reference light and the signal light that have passedthrough the second polarization beam splitter onto a single side of theholographic information storage medium.
 46. A method of recordinginformation to a holographic information storage medium, the methodcomprising: emitting a reference light and a signal light, each beinglinear polarized and orthogonal to each other; guiding the emittedreference light and the emitted signal light so that the reference lightand the signal light cross each other at a first crossing point afterpassing through different optical paths; uniting the optical paths ofthe reference light and the signal light; and illuminating the referencelight and the signal light onto a single side of the holographicinformation storage medium.